Once you have both accounts set up, you will see two accounts in your Thunderbird sidebar. They will both display your SEAS email address, but the one with a subfolder called "[Gmail]" is your Gmail@SEAS account.

Move email by dragging and dropping emails or whole folders from your SEAS email to your Gmail@SEAS account. Hold down the "Ctrl" key on your keyboard to copy email rather than moving it. Drag folders onto the "[Gmail]" folder.

Before

After

Once you have copied over your email, you can remove either or both accounts by going to the Tools menu, and selecting Account Settings...